This invention relates to the calculation of the temperature of engine lubricating oil without direct measurement thereof based on measurement of other engine parameters in engine operation. It particularly relates to the use of such a calculated engine oil temperature in an electronic indicator apparatus for providing the operator of an engine driven motor vehicle with information pertaining to when the engine oil should be changed.
It is well known in the motor vehicle art that engine durability is directly related to the lubricating ability of the engine crankcase oil, and that its lubricating ability becomes degraded with engine operation and time. Thus, most engine and vehicle manufacturers provide their customers with guidelines for determining when the engine oil should be changed.
By way of example, the General Motors Corporation Maintenance Schedule for gasoline fueled passenger cars and light trucks recommends that, barring severe operating conditions, the engine oil should be changed every 7,500 miles (12,000 km) or 12 months, whichever comes first. Under severe operating conditions, however, the schedule recommends that the engine oil be changed every 3,000 miles (4,800 km) or 3 months, whichever comes first. In this regard, severe operating conditions are defined as including trips less than 4 miles (6.4 km) in below freezing weather, extended idling, trailer towing, operating in dusty areas, and extended stop-and-go driving.
Those skilled in the art recognize that oil change maintenance schedules of the type set forth above are only guidelines, and that depending on the engine operating conditions, the required oil change interval may be as short as 1,700 miles (2,720 km) or as long as 10,000 miles (16,000 km). Accordingly, it has been proposed to provide an estimate of the required oil change interval for a given vehicle by electronically monitoring certain key engine operating parameters in the course of vehicle operation between oil changes. When it is determined that an oil change is required, the operator is so informed by an instrument panel indicator. In one system, for example, the actual mileage of the vehicle is multiplied by a factor, the value of which is varied according to the operating speed of the engine, and an engine load indication (such as coolant temperature) to arrive at a calculated mileage. When the calculated mileage exceeds a threshold, an indicator is tripped to inform the operator that an oil change is required.
Research in this area has shown that a more direct and accurate indication of the oil degradation, and therefore the need for an oil change, can be determined by monitoring the temperature of the oil, without regard to engine loading or operating conditions which are indirectly related to oil temperature variations. Excessive degradation of the engine oil occurs at its temperature extremes. At high oil temperatures, antioxidants in the oil tend to become depleted, and the oil becomes more viscous and acidic due to oxidation and nitration. In addition, insoluble particles are deposited on the engine surfaces as a varnish or sludge. At low oil temperatures, fuel, water and soot tend to accumulate in the oil, reducing its viscosity and increasing wear. In addition, acids produced by incomplete combustion reduce the ability of the oil to prevent rust and corrosion. This research led to the invention of an automatic engine oil change indicator system as claimed and described in U.S. Pat. No. 4,742,476, issued May 3, 1988 to Schwartz et al. The system of this patent used an engine oil temperature sensor in the crankcase to specifically measure engine oil temperature and provide a signal thereof as the primary sensed input to the control system. If there is no other function served by an engine oil temperature sensor, however, it would be desirable for reduced cost to provide such a system in which the engine oil temperature may be accurately estimated from the signals of other sensors already present.